The present invention relates to an improved topology for single and multi-phase power factor correction and improved total harmonic distortion (THD).
Electric power distribution is a necessary element of systems that operate with electronic power or in the distribution of electronic power. Electronic devices are generally connected to some power source wherein the power arrives in one form and is transferred and transformed into a form more suitable for the operation of the equipment.
Power is more efficiently transferred in AC form with most utilities providing AC sources. For devices requiring DC input, rectification of the AC source to DC is required. AC-DC converters may also be used to xe2x80x9cactivelyxe2x80x9d rectify and boost the resulting DC output. Power converters, such as inverters, are necessary in modem power systems for converting AC or DC power to conditioned AC for feeding a power grid or for direct connection to loads. The AC input power may come from any of the energy generating devices such as photovoltaics, micro-turbines, fuel cells, superconducting storage, wave energy, etc. . . . Modern systems need to be able to interconnect a variety of sources and provide stable power.
An example of a common inverter device is a half bridge circuit configuration. An AC input source connects to a full-wave rectifier with a smoothing capacitor connected to a DC output. Across the smoothing capacitor, a series circuit of switching elements is connected while these switching elements are turned alternately ON and OFF at a high frequency by a pulse width modulator (PWM). Across one of the switching elements, a series resonance circuit of a resonating inductor and a resonating capacitor is connected through a DC component cutting capacitor, while a load is connected in parallel across the resonating capacitor.
A DC voltage is generated at the smoothing capacitor so that the switching elements are alternately turned ON and OFF, with a high frequency rectangular wave voltage V is applied through the DC component cutting capacitor to the load, and a high frequency voltage is supplied to the load due to a resonating action of the resonating inductor and capacitor.
The resulting switching has an inherent inefficiency. The switching elements can be power MOSFET or IGBT, with the switching controlled so that an inverter circuit current will be at a delayed phase with respect to the high frequency rectangular wave voltage V. The power factor of the inverter circuit current is, therefore not unity with respect to the high frequency rectangular wave voltage. A larger current than that to be supplied to the load causes a switching loss. There are further problems because an xe2x80x9cactivelyxe2x80x9d controlled switching element of a large current rating is required, and a high cost incurred.
Another example of a power source device is a full-wave rectifier that connects to an AC input source with one of the switching elements connected through an inductor to the DC output of the full-wave rectifier. A smoothing capacitor is connected through a diode across the switching element. An input current in accordance with the input voltage from the AC power source is supplied. In this case, a voltage boosting chopper circuit is established by means of the inductor, one switching element, diode and smoothing capacitor, while the one switching element is also employed as a switching element of the inverter circuit.
To the smoothing capacitor, a series circuit of a pair of switching elements is connected, and a diode is connected in inverse parallel across each of these switching elements. Across one of these switching elements, an inverter load circuit is connected through a DC component cutting capacitor, and the inverter load circuit includes a series resonance circuit of another resonating inductor and a resonating capacitor, while a load is connected in parallel across the resonating capacitor. The respective switching elements are caused to be alternately turned ON and OFF by a DC voltage from the smoothing capacitor, and a rectangular wave voltage is supplied to the inverter load circuit, whereby a high frequency is caused to flow to the resonating inductor. The switching element also acts as a chopper circuit, so that an input current will be caused to flow through one of the inductors, the input current distortion is improved, and the smoothing capacitor is charged by an energy accumulated in the inductor.
It should be understood that a current from the resonating inductor and a current from the other inductor are made to flow to the switching element as superposed on each other so as to be a large current. This results in a power loss or inefficiency as the switching element has to be larger in size to handle larger loads.
Another example is a capacitor that is connected in series with an inductor, whereby the charging energy to the smoothing capacitor is weakened, and the voltage of the smoothing capacitor is restrained. The chopper operation turns xe2x80x98ONxe2x80x99 the switching elements which causes an input current to flow from the AC power source through the full-wave rectifier, another capacitor, the inductor, a switching element and full-wave rectifier, and an energy is accumulated in these another capacitor and one inductor.
As the switching element turns xe2x80x98OFFxe2x80x99, a current flows through the inductor, the diode, smoothing capacitor, full-wave rectifier, another capacitor and inductor, and the smoothing capacitor and capacitor are charged by a voltage induced at the inductor. Further, when the other switching element is turned xe2x80x98ONxe2x80x99, a current flows through another capacitor, another diode, another switching element, an inductor and another capacitor so that the other capacitor will be a power source, and a current in a reverse direction to that in the previous period is caused to flow to the one inductor.
Pulse Width Modulated (PWM) power inverters are generally available in three-phase bridge, H-bridge, and half bridge configurations. The rectifier fed, electrolytic bus capacitor banks often consist of two or more capacitors connected in series to expand the maximum bus voltage capacity. For distributed power applications a neutral is typically connected to the center of the DC bus, between the two series caps. The capacitor charge path of the PWM inverter is through the series capacitors simultaneously, tending to keep the total bus voltage (upper and lower bus voltages) constant.
However, a diode rectifier circuit such as is typically used by a switching power supply requires a large input current value relative to the power consumption as represented by an input power factor of about 0.6 to 0.67. Thus, the reactive power in supplying and distributing power systems is generally inefficient, as well as very high THD.
There have been numerous attempts to alleviate the problems and inefficiencies noted in the prior art devices. FIG. 1 shows a basic schematic of a three phase AC input with a single boost. The inductor L is connected in series after the rectifier section 10 so that it operates on the rectified three phase AC signal. When the gating switch SW is turned xe2x80x98ONxe2x80x99, the current builds up in L. When the switch SW is turned xe2x80x98OFFxe2x80x99, the inductor charge L is discharged into the capacitor C and forms the output DC signal. Typically the switch SW is pulse width modulated to control the energy transfer.
FIG. 2 illustrates a basic schematic of a three phase four wire scheme with dual boost. Once again, the inductors L1 and L2 are connected after the rectifier section 20. The switches SW1 and SW2 control the current flow and charging of the inductors L1 and L2 that are discharged into C1 and C2 respectively with the DC output level formed from the output capacitors C1 and C2.
The basic schematic of FIG. 3 illustrates a three phase active converter. In this circuit, the AC inductors L1-L3 are each connected to the switches SW1-SW6, wherein a complicated switching control operates to xe2x80x9cactivelyxe2x80x9d rectify and boost the resulting DC voltage that is stored in the output capacitor C. The three-phase bridge, active converters (typically 4 quadrant) are used in a variety of applications, including regenerative motor drives to accomplish PF correction. However, these topologies carry a high cost using six fully rated IGBT switches that operate at a relatively low efficiency. Further, these AC-DC power converters require complex control techniques and lack reliability.
A three phase rectifier circuit is also described in patent application PCT WO 01/69770. This invention describes a three phase rectifier with a low frequency filter that includes two AC reactors. This topology does not include boost switches and the filter capacitors are xe2x80x98afterxe2x80x99 the three phase reactor.
The circuit of U.S. Pat. No. 6,049,472 is a single phase circuit application for a power factor improving circuit operating without a boost switch. The primary focus of the xe2x80x98this patent is the second stage of the circuit.
U.S. Pat. No. 6,028,776 (""776) discloses a power factor correction circuit having a double stage layout with a bridge diode, booster, a switch section, a control section, a delay section, and an output section. The ""776 invention is a single phase implementation that does not account for the difficulties in multi-phase operations, and uses DC inductors.
What is needed is a power factor correction scheme that eliminates the problems of the prior art devices. There is a current need for a robust, simple and low cost single or poly-phase power factor correction topology in the energy storage/power fields. Such a system should allow ease of installation into existing systems and replace only a front end section. Any such improved power factor correction scheme should also reduce complexity, cost, and space requirements in addition to improved efficiency.
The present invention has been made in consideration of the aforementioned background. An improved, low cost, high-efficiency method for improving single or poly-phase input power factor and total harmonic distortion. The performance is superior to prior art DC inductor based PF correction circuits.
The present invention provides an efficient and reliable method for improving three phase input power factor and THD applicable to power supplies, motor drives, power conditioners, power generation equipment, hybrid power conditioning and distribution equipment, appliances, and flywheels.
An object of the present invention is an improved circuit design ideal for single boost, split phase dual boost topologies, and other poly-phase systems. The implementation is also flexible, utilizing a wide range of input voltages and currents (including synthesized input voltage waveforms other than sinusoids), and variable frequency voltage sources such as flywheels, switches, such as IGBT, MOSFET, in combination with AC inductors and typical high frequency rectifier diodes, high frequency rectifiers, or even some line frequency rectifier diodes.
In one embodiment the present invention comprises a three-phase six pulse diode rectifier and a boost switch(es) with a three-phase reactor or three AC line inductors to provide DC volts regulation and input PF correction and THD reduction.
While there are three-phase power active converters (4 quadrant usually) used in regenerative motor drives that accomplish PF correctionxe2x80x94they do so at a very high cost and require six switches (usually IGBT""s) and with low efficiency.
An object of the invention is an AC voltage to DC voltage converter with improved power factor, the converter comprising a single phase AC input with a pair of inductors each coupled in series to the AC input on a first end of the inductors. There is a rectifier section coupled to the inductors on a second end of the inductors, wherein the rectifier section converts the AC voltage to the DC voltage. A switch is coupled in parallel to the rectifier section for modulating stored charge from the inductors to an output capacitor, with a diode connected in series to the output capacitor.
A further object is the AC voltage to DC voltage converter wherein the switch is coupled to a pulse width modulating controller. In addition, wherein the pulse width modulating controller uses a DC output volts feedback to provide boost synchronization of the AC input.
Yet another object is the AC voltage to DC voltage converter, wherein the pulse width modulating controller controls the switch to maintain the AC input within an upper tolerance band and a lower tolerance band. In addition, wherein a switching frequency from the pulse width modulating controller varies as a function of the input volts with a minimum di/dt and maximum switching frequency occurring at a zero crossing.
An additional object is the AC voltage to DC voltage converter, further comprising a processing section, wherein the processing section calculates a switching frequency correction factor using a preset and calculated tolerance band that tracks the input volts and based on the L-N voltage at the zero crossing.
An object of the invention is a circuit for converting an AC voltage into a DC voltage, the circuit comprising a three phase AC input, three AC inductors having a first end and a second end, wherein the inductors are coupled in series on the first end to the AC input. A rectifier section coupled to the inductors on the second end, wherein the rectifier section converts the AC voltage to the DC voltage. There is a switch transferring stored charge from the inductors to an output capacitor through a series connected diode, with a pulse width modulator section controlling the switch.
A further object is the AC voltage to DC voltage converter, further comprising a switching ripple filter section coupled to the three phase AC input.
And an additional object is a circuit for converting an AC voltage into a DC voltage, the circuit comprising a three phase AC input with three AC inductors having a first end and a second end, wherein the inductors are coupled in series on the first end to the AC input. A rectifier section coupled to the inductors on the second end, wherein the rectifier section converts the AC voltage to the DC voltage. There is at least one switch transferring stored charge from the inductors to at least one output capacitor through a series connected diode, and a pulse width modulator section controlling the switch.
Other objects, features and advantages are apparent from description in conjunction with the accompanying drawings.